1. Field of the Invention
The present invention relates to a sequential scanning converter for converting a picture signal of interlaced scanning to a sequential scanning scheme by scanning line interpolation, and displaying it, and particularly, to a sequential scanning converter which improves the picture quality degradation caused by scanning conversion.
2. Related Art of the Invention
The current television-system employs an interlaced scanning scheme. In the interlaced scanning scheme, two fields, odd and even fields, form one frame. Thus, for instance, in the HDTV system, the number of scanning lines per frame is 1,125, but, if the viewing distance becomes short, the coarse scanning line structure forming each field becomes visible, causing degradation in the picture quality.
As a method for improving the scanning line structure, there is a scheme in which a video signal is interpolated between the upper and lower scanning lines in a field so as to be converted to the sequential scanning, but the frame made by this has a problem that the sharpness of the vertical direction decreases to affect the picture quality.
As a conventional sequential scanning converter, to solve the above problem, for instance, a sequential scanning converter for improving the picture quality degradation due to the sequential scanning by simple two scans through controlling the brightness of the interpolation signal for sequential scanning according to the brightness of the picture signal (for instance, "Sequential Scanning Converter Circuit," "Japanese Patent Application Laid-Open No. 63-179685), or a sequential scanning converter for preventing apparent degradation of the sharpness by decreasing the amplitude of the interpolation signal after sequential conversion (for instance," "Television Display of Clearvision System," "Japanese Patent Application Laid-Open No. 4-95479)" was filed.
However, in the above conventional arrangement, a quasi-interlace effect is obtained by decreasing the amplitude-of interpolation lines in the conversion to the sequential scanning as compared with the amplitude of real lines, thereby to enable an apparent increase in the sharpness of the vertical direction, but, it has a problem that, since there is a difference between the amplitudes of real and interpolation lines for a signal having no gradation such as an animation created by computer graphics (hereinafter referred to as CG), the occurrence of streaks greatly degrades the picture quality. In particular, the picture quality degradation becomes remarkable as the screen becomes larger and larger in recent years.